1. Field of the Invention
The present invention relates to a coated metal sheet that can be used as a battery container for alkaline-manganese batteries, and to battery containers or batteries produced therefrom.
2. Description of the Background Art
Smaller internal resistance is preferred in dry batteries, especially those used as a power supply for a strobo flash in photography where a large current is to be instantly discharged. For an alkaline-manganese battery, the contact resistance between the positive electrode mix, formed into pellets mixed with manganese dioxide as the active material and graphite powder as the conductive material, and the battery container, referred to as the positive electrode can, is the largest of all the internal resistances.
Japanese Patent Laid-open Sho 60-240056 discloses a way of diminishing the contact resistance by forming a conductive coating on the inner surface of the battery container to raise the ratio of active material/conductive material in the positive electrode mix and thereby increase battery life.
However, there are some difficulties encountered in forming a conductive coating on the inner surface of a battery container. As an example, the conductive paint is generally applied by a spraying operation which results in clogging of the spray nozzle or the application of an uneven conductive coating thickness. Furthermore, even if these difficulties can be overcome, the inner surface of every small battery container still needs to be coated with conductive paint, and thus, the poor production efficiency of such a process is reflected in an increase in production cost.
In order to overcome the above-mentioned difficulties, a process involving the coating of a conductive paint on a surface-treated metal sheet, and the subsequent formation of the coated surface-treated steel sheet into a battery container is used in some battery industries, but has not yet succeeded on an industrial scale. One reason for this is that it is difficult to form a battery container from the coated metal sheet without the conductive coating peeling off during the process.
There are several sizes of alkaline-manganese batteries on the market, and some of them are formed by using a severe process. For example, the container for the LR6 dry battery is formed by the following process:
At first, a blank is punched out from a coated metal sheet. Then, the blank is successfully formed into a can having a smaller diameter by drawing (8 to 12 step procedure in the process). The finished battery container has an inner diameter of 13.8 mm, and the total drawing ratio (blank diameter/inner diameter of finished drawn can) becomes 4.2. It is essential that the formability of the coating is satisfactory along with the coating having the property of excellent adhesion to metal sheets such that the coating does not peel off the metal sheet after being formed by such a severe process.
Thermosetting resins, such as epoxy resin, are used as the main adhesive in conductive paints available on the market for coating the inner surface of a formed battery container. As long as the paint is sprayed on to coat the inner surface of the battery container, and the container is used without further forming, then there is no peeling off of the coating. However, when a metal sheet, previously coated with a conductive paint is drawn, cracks are produced in the coating during forming as the thermosetting resin is generally lacking in formability, and a part of the coating peels off the metal sheet where the peeled coatings become built up in the forming mold, which promotes further peeling off of the coating or sometimes causes damage to the mold. In order to avoid a build up of peeled coatings, which causes the poor production efficiency, the peeled coatings need to be wiped off the mold frequently.
Japanese patent Laid-open Hei 6-342653 discloses a way of reducing the peeling off of coating during the drawing of the coated metal sheet, which involves dipping a metal sheet into chromate solution containing carbon black to provide conductivity to the insulated chromate film, forming a chromate film to improve coating adhesion, and then coating with conductive paint containing epoxy resin, etc. Insofar as a thermosetting resin such as epoxy resin is used as the main component of paint, the poor formability of the coating cannot be overcome and the cracks formed in the coating during the drawing of coated metal sheet cannot be avoided. It is also supposed that the perfect rejection of peeling off of coating is hardly performed.
There is another reason why the battery industry is hesitant in undertaking development of a conductive film coated metal sheet which can be formed by drawing. During the initial stages of developing alkaline-manganese dry batteries, great effort was directed towards preventing the so-called creep phenomenon, a phenomenon caused by the leakage of potassium hydroxide electrolyte from the sealing part of the battery container and cap, and in which more potassium hydroxide electrolyte leaked upon evaporation of moisture from the system. In the alkaline-manganese dry battery on the market, the upper part of its inner surface, which corresponds to the sealing part, is designed not to be coated with conductive paint so as to avoid the creep phenomenon which occurs when conductive paint is applied as a coating on the inner surface of the battery container. Conductive paint can be easily coated on the inner surface of every battery container that is kept partially uncoated in the manner described. When a blank is punched from a coated metal sheet and made into a battery container by drawing, it is extremely hard to coat the conducting paint on a wide metal strip with only on the section corresponding to the sealing part on the inner surface of the battery container being kept uncoated.
On the other hand, polyvinyl butyral as a thermoplastic resin and dibutyl phthalate as a plasticizer are respectively used as a binder for ceramic particles and as a plasticizer when ceramic articles are formed. These compounds are also used in the battery industry.
Japanese patent Laid-open Hei 3-291850 discloses a method of producing positive electrode material of a spiral type lithium battery which involves mixing polyvinyl butyral and dibutyl phthalate with manganese dioxide and graphite powder, and then uniting them with a collector by extruding the mixture and forming a sheet electrode. In this Japanese patent, the inventors of the present invention had formulated a paint, consisting of polyvinyl butyral, dibutyl phthalate, conductive material and solvent, and applied it as a coating on a metal sheet. The coated metal sheet was then formed into a battery container by drawing. The resulting coating film had excellent formability and was drawn comparatively well. However, it had poor adhesion properties and the coating film peeled off at the part known as the ear and is to be cut off at the final forming stage when the coated metal sheet is drawn. When battery containers are continuously mass-produced by drawing, the peeling off of the coated film is undesirable as the peeled films become built up on the drawing mold. In addition, the hardness of the coated film was such that scars easily resulted when the coated metal sheet was drawn or pellets of the positive electrode mix were packed into battery container.
As mentioned above, there is no ready-made or known conductive paint which have the properties of excellent conductivity, formability and adhesion which are the objective of the present invention.
Accordingly, the object of the present invention is to produce a coated metal sheet having excellent formability and adhesion properties when drawn into a battery container, and does not result in peeling of the coated film after drawing. The coated metal sheet also is resistant to the alkali electrolyte packed into the battery container, has excellent conductivity and is sealable at the sealing part upon being formed into battery container.